Hearing a jackhammer like sound in my house around the same time once a week

I am hearing a jackhammer like sound in my house. It's happened twice now. One per week around the same time. It happens from 6:20AM and stops around 6:35AM. It pulses for about 3 seconds, then stops. Then repeats in about a minute. It's louder in the upstairs on my house. Sounded like it was coming from the attic so I looked around up there--heard the sound, but it didnt sound any louder, nor could i identify it.